1. Field of the Invention
Example embodiments relate to a conveying device that conveys a sheet bundle formed by stitching sheet-like recording media such as paper sheets, recording paper, and transfer paper (hereinafter, simply referred to as “sheet”), and by folding the stitched sheet-like recording media; a spine forming device provided with the conveying device and a spine forming unit that forms a back portion of the sheet bundle in a flat shape; and an image forming system provided with the spine forming device and an image forming device.
2. Description of the Related Art
In saddle stitch bookbinding that is widely used as simple bookbinding, the need to make the height of a fold (bulge) smaller after bookbinding is very high. Bound booklets are usually handled in stacks of dozens for transportation, delivery, and the like. However, each of the half folded and saddle stitched booklets has a bulge, which makes them unstable and easy to topple when they are stacked.
More specifically, when a sheet bundle is saddle stitched and folded in the middle (double fold), the double folded sheet bundle is likely to have a bulge in the thickness direction near the folding portion and look unattractive. In addition, when the sheet bundle has the bulge near the folding portion, the back side becomes thicker while the fore-edge side is thinner as a booklet. Accordingly, when the sheet bundles are stacked facing the same direction, the stack becomes more lopsided as more booklets are stacked. Consequently, when a large number of sheet bundles are stacked, the stack topples because of excessive tilting, making it difficult to stack a large number of sheet bundles.
In contrast, when a booklet is formed with the folding portion of a double folded sheet bundle flattened like a spine, the bulge of the booklet is pressed, whereby a large number of such booklets can be stacked. In other words, even a couple of booklets with a bulge topple easily when stacked on a desk, creating a problem in handling such as storage and transportation. When a back portion corresponding to the folding portion is flattened, the bulge can be reduced to a bare minimum, thereby resolving the problem. The back portion here means the portion including a spine that is a back face and portions of a front cover and a rear cover connecting to the spine (hereinafter, referred to as back-face portion), and corresponds to the portion opposite to the fore edge.
As for the techniques specific to flattening a booklet, inventions disclosed in the following three patent documents, for example, are known. In the invention disclosed in Japanese Patent Application Laid-open No. 2001-260564, a front cover and a rear cover of a pamphlet composed of a sheet bundle folded so that a back portion has a curvature are fixed by clamping adjacently to the back portion with a pressing unit, and a forming roller is applied once or more along the lengthwise direction of the projected back portion with a pressure sufficient to smooth out the curvature to flatten the back portion.
Japanese Patent Application Laid-open No. 2007-98874 discloses that, similarly to the invention disclosed in Japanese Patent Application Laid-open No. 2001-260564, a folding portion of a saddle stitched booklet is clamped and a back face of the booklet is pressed by moving a roller to flatten the folding portion so as to reduce the height of the fold. However, when delivering the flattened booklet, the booklet is conveyed downstream by an ordinary roller pair.
These inventions have an effect of flattening the curvature of the back portion. However, because a face is formed at the back portion of the booklet by locally and continuously pressing the back portion with a pressing roller, wrinkles and tears are likely to occur on the back face and in the stitched portion. In addition, the roller is moved along the folding portion, resulting in a long forming time.
Japanese Patent Application Laid-open No. 2007-237562 discloses a sheet bundle forming device including: a clamping unit that clamps a portion other than a periphery of the back portion of a folded sheet bundle in the thickness direction; a back portion pressing unit that presses the back portion in the direction toward the fore edge and opposite to the back portion by pressing a back portion pressing face formed on a back portion pressing member to the back portion of the clamped sheet bundle; and a compressing unit that forms the periphery of the back portion of the sheet bundle by compressing the periphery of the pressed sheet bundle in the thickness direction. In this invention, the compressing unit compresses the portions of the front cover and the rear cover near the fold having a bulge resulted by pressing the back portion in the fore edge direction by the back portion pressing unit, in an attempt to reduce the bulge.
However, as in the invention disclosed in Japanese Patent Application Laid-open No. 2001-260564, when the projected portion is partially deformed by the contact of the roller, wrinkles and the like tend to appear in a direction perpendicular to the fold, giving it an unattractive appearance. Furthermore, when the size of the sheet is larger, because of the traveling time of the roller required, productivity may deteriorate depending on the number of sheets in a bundle. More specifically, due to the recent circumstances of energy conservation, in a device of this type, it is important to obtain an energy saving effect by efficiently operating the device. Generally, when efficiency is considered, the processing conditions such as the pressure applied and the amount of repetitions differ depending on the number of sheets, paper thickness, and paper type. However, in work using a roller as disclosed in Japanese Patent Application Laid-open No. 2001-260564, the fact that the number of repeats of the roller run is the only option available leads to a problem in that the work cannot be performed under the most efficient condition.
In the invention disclosed in Japanese Patent Application Laid-open No. 2007-98874, because the booklet is conveyed by the ordinary roller pair at the time of delivery, the leading end (stitched portion) of the flattened booklet is gripped by the nip of the roller pair. Consequently, the folding portion is deformed when gripped by the nip, and in the worst case, wrinkles, smudges, and the like appear on the flattened portion, deteriorating the quality of bookbinding. Particularly, when the booklet has a large number of sheets and is thick, the contact angle with respect to the outer circumference of the roller becomes large, making the damage to the back-face portion of the booklet more prominent. To prevent this, the diameter of the roller could be increased so as to make the contact angle smaller. However, when the diameter of the roller is increased, a space is required to accommodate that increased diameter roller.
In the invention disclosed in Japanese Patent Application Laid-open No. 2007-237562, the occurrence of wrinkles and tears can be reduced and the face can be formed on the back portion of the booklet. However, to press the back portion in the fore edge direction and then to flatten the front cover and the rear cover by compressing them with the compressing unit, the multiple units of the clamping unit, the back portion pressing unit, the compressing unit, and the like need to be operated in sequence after the booklet is stopped by abutting on a stop plate. This results in an insufficient effect of shortening the working time.